1. Field of the Invention
The present invention relates to a device for removing a floating cell by using generic flow control (GFC) which is generated when a ring type subscriber network is formed to provide broadband services to small offices or home in a Broadband Integrated Services Digital Network and a method thereof.
2. Discussion of Related Art
With development of communication technologies, the demand for high quality regarding various communication services has increased. To satisfy these demands, an asynchronous transfer mode (ATM) Broadband Integrated Services Digital Network (B-ISDN) has been proposed to provide high speed data service including transmission and burst service such as an actual time video service with variable bit rates, as well as continuous services such as a voice service or a video service.
In the past, development has been focused on large-scaled switches applicable to national key networks and the medium-scale of devices for supplying broadband services to private networks or a plurality of regions rather than on the devices for directly supplying services to subscribers.
Most of the above devices are structured in a switch type and thus are adequate to traffic transmission/reception between regions requiring a large input/output capacity. However, particularly when these are used at home or small offices using direct services, these are disadvantageous as compared to a ring or a bus type multi-access configuration, as described in the following.
First, in the switch-structured device, each service terminal must be connected to the switch via its own transmission line, and this requires more transmission lines as compared with the case of a ring or a bus type device, causing a substanial increase in cost.
Secondly, with a complicated switch structure, there is difficulty in maintenance or operation in comparison with a simple multiplexer or a ring type device.
Thirdly, when an input port is required to be added, the multi-access configuration can simply add the input port by connecting it to the preceding terminal or inserting it in the existing configuration. However, the switch-structured device must install it in a switch unit, causing unnecessary port consumption.
However, even when a user turns off the power of any terminal, the switch-structured network does not affect the traffic flow of other terminals. To the contrary, the ring type network cannot communicate any more via the terminals placed in the downstream of the powered-off terminal.
Thus, a power off bypass switch which can control the data flow even at the power off is required for each terminal.
In addition, there is required a method for efficiently removing floating cells generated due to the power off of any terminal during a communication and thus preventing unnecessary cells from occupying the bandwidth.
FIG. 1a and 1b are diagrams showing general configurations of a switch-structured network and a ring type network.
In a switch-structured network of FIG. 1a, each terminal is connected to a broadband network terminator 11 via its own transmission line. Therefore, even when a terminal is powered off during communication, this does not affect the other terminals nor occupy the bandwidth of the other terminals.
Meanwhile, in the ring type network of FIG. 1b where each terminal is connected to a broadband network terminator 16 via one shared line in ring form, if user voluntarily powers off any of the terminals 17, 18, 19 and 110, all terminals placed downstream of the powered-off terminal cannot communicate any more.
Thus, even though a power off bypass switch is installed to prevent the problem described above, all cells to be transmitted to the terminal continuously occupy the bandwidth of the ring. These cells are called floating cells.
Generally, in order to control the flow between each terminal and a network terminator in a point-to-point connection network, four bits are allotted to a generic flow control field in an asynchronous transfer mode (ATM) cell header. But in most cases, the generic flow control field is not used.
FIG. 2 is a diagram showing a cell header format designated in a conventional private network. In FIG. 2, 4 significant bits of the first byte in the cell header comprised of five bytes are assigned to the generic flow control field, and a virtual path identifier (VPI) and a virtual channel identifier (VCI) determine the path via which the cell is to be routed.
The generic flow control field used to control the traffic amount input to the network termination device 11 has a control mode and a non-control mode.
In the non-control mode, the flow of the data input to the network termination device is not controlled and a value "0" is input to the generic flow control field.
On the contrary, the control mode gives a priority order to each cell to be input to the broadband network termination device 11 and then permits the input of the cell of higher priority first, or temporarily suspends the cell flow in the terminal unit (12, 13, 14, 15) if the sum of the traffic sent toward the network termination device 11 exceeds the network termination device capacity.
However, with a complicated algorithm and a greater hardware requirement to control the traffic in the terminal unit, none of existing terminals and network termination devices support the control mode.
In addition, since it is not easily applicable to the multi-access type network, all devices support the non-control mode.